This application is a national stage application filed under 35 U.S.C. 371 of International Application No. PCT/FR2008/000595, filed Apr. 25, 2008, which claims priority from French Application No. 07/03093, filed Apr. 27, 2007.
The invention relates to a device for mass storage of information, and also to a method of storing information with the help of said device.
Numerous techniques are known in the prior art for mass storage of information. Amongst the techniques in most widespread use, mention can be made of non-volatile semiconductor memories, magnetic tapes or disks, and optical disks.
Present-day (magnetic) hard disks present storage density of the order of 100 gigabits per square inch (Gbit/in2) where 1 Gbit=109 bits. Higher densities (up to 400 Gbit/in2) can be achieved in the laboratory with the help of the perpendicular magnetic recording technique. Conventional optical disks present lower recording densities, with the exception of holographic disks, but they are still at an experimental stage, and in any event they are not rewritable.
The techniques that appear to be the most promising for obtaining ultrahigh information storage densities, of the order of 1 terabit per square inch (Tbit/int) or more, where:
1 Tbit=103×Gbits=1012 bits
are based on using microtips and they are derived from scanning microscope technologies such as atomic force or tunnel effect microscopy. The article by D. Saluel and J. M. Fedeli entitled “L'enregistrement ultra haute densité device micropointes” [Ultrahigh density recording with microtips], published in Signaux No. 94, September 1999, pp. 9 to 22, summarizes those techniques that are based on principles as varied as storing charge, changing phase, and magnetic recording.
An object of the invention is to provide a novel technique for mass storage of information at ultrahigh density, the technique being of the microtip type, based on making use of an electrochemical effect. This technique, which is relatively simple to implement, may advantageously be reversible, which means that it is possible to delete information that has been recorded and that the recording medium can be reused an indefinite number of times.
More precisely, the invention provides a device for mass storage of information, the device comprising: an information storage substrate; an electrically-conductive tip for atomic force microscopy located above a surface of said substrate, in electrical contact therewith; and means for applying a potential difference between said tip and said surface of the substrate; the device being characterized in that: said surface of the information storage substrate is constituted by a material presenting electrical conductivity that is both electronic and ionic in nature; and in that said means for applying a potential difference between said tip and said surface of the substrate are adapted to apply a potential difference that is sufficient to induce a redox reaction of said material, modifying its electrical conductivity.
In particular embodiments of the device of the invention:
The invention also provides a method of using a device as defined above for mass storage of information, the method comprising: a step of writing information by applying, between said electrically-conductive tip for atomic force microscopy and said substrate, a potential difference that is sufficient to induce a redox reaction of said substrate, thereby modifying its surface electrical conductivity; and a step of reading the information as recorded in this way by measuring the surface electrical conductivity of said substrate by means of said electrically-conductive tip for atomic force microscopy, or of a second such tip, said measurement comprising applying a potential difference that is not sufficient for inducing a redox reaction of said substrate that would modify said surface electrical conductivity.
In particular implementations of the method of the invention:
Other characteristics, details, and advantages of the invention appear on reading the following description made with reference to the accompanying drawing given by way of example, and in which:
a, 3b, and 3c show an example of the method of using the
The device of
The tip 10 is located above the surface 31 of a storage substrate 30 at a distance therefrom lying in the range 0 (direct contact between the tip and the surface) and about 100 nanometers (nm). A water meniscus 20 forms spontaneously between the tip 10 and the surface 31 by condensation of ambient humidity, and this occurs even in an atmosphere that is very dry. The meniscus 20 provides electrical contact between the tip 10 and the surface 31 and it constitutes an electrochemical cell of nanometer size with the surface and the tip constituting the electrodes thereof.
The storage substrate 30 is essentially constituted by a material presenting electrical conductivity of a nature that is both electronic (like a metal) and ionic. A material of this type is generally constituted by associating a metal oxide with an alkali metal: by way of example it may be one of the materials used for making lithium batteries. More precisely, such materials can be characterized by the general formula:
Ax(M1)v(M2)w(M3)y(M4)zBβ
where:
The parameters x, v, w, y, z, and β must satisfy the following inequalities:
Furthermore, x needs to take a value serving to ensure chemical stability of the compound (the stability ranges for the parameter x depend on the qualitative composition of the material).
The parameters x, v, w, y, z, and β also need to satisfy the following equalities:
v+w+y+z=1 and x+v+2w+3y+4z−2β=0
Advantageously, the material of the recording medium 30 may be characterized by the general formula A′xMyBβ where:
Particular mention may be made of the following materials: NaxCoO2, LixCoO2, LiXNiO2, LixMn4O9, LixTiO2, Li4+xTi5O12, LixV2O5, LixV6O13, LixNiyCo(1−y)O2, LixFeO2, LixMnO2, LixMn2O4, LixMoO2, and LixNi1/3Mn1/2CO1/3O2.
NaxCoO2, LixCoO2 are particularly preferred.
The material may be hydrated, i.e. it may include water incorporated in its crystal structure, however that is not generally desired since the presence of water molecules runs the risk of impeding the mobility of alkali metal ions. As shown below, this mobility is essential for the operation of the invention.
It is advantageous for the substrate 30 to be monocrystalline.
As shown in
The materials constituting the substrate 30 are generally unstable in air. In particular, NaxCoO2 tends to oxidize, and also to react with CO2 to form insulating carbonates. It is therefore necessary to maintain the substrate 30 in a protective atmosphere, e.g. by enclosing it together with the tip 10 in a sealed housing 50.
A voltage generator 41 serves to establish a time-varying potential difference Ve(t) between the substrate 30 and the tip 10. The electric circuit 40 is closed by the water meniscus 20, thereby allowing an electric current i(t) to pass.
This effect is reversible: applying a potential difference of opposite polarity (reducing) causes the sodium to go from the surface into the body of the substrate, thereby decreasing the electrical conductivity thereof.
More generally, the principle of the invention is to make use of the existence of a plurality of stable redox states in the material of the substrate 30, which states are associated with structural changes that are preferably reversible, and with modifications that are also reversible to the electrical properties of said material.
By way of example, applying an oxidizing potential difference of +2.5 V to a substrate of NaxCoO2 with x=0.75 gives rise to a local reduction in the parameter x, which in turn induces an increase in conductivity of about two orders of magnitude.
Applying a potential difference that is too small gives rise to hardly any or no variation in conductivity, whereas too great a potential difference (3 V or more maintained for longer than a few seconds with NaxCoO2) gives rise to variations that are not reversible. The range of voltage values that can be used for reversibly writing information depends on the material under consideration, but, generally speaking, its absolute value lies in the range 0.5 V to 10 V, approximately.
Information can thus be written on the surface 31 in the form of variations of electrical conductivity. The information as recorded in this way can be read, e.g. by an ammeter 42 connected in series with the voltage generator 41. During an information-reading stage, the generator 41 applies a potential difference Vr between the substrate 30 and the tip 10 that is not sufficient to induce the above-described electrochemical reaction, and the ammeter 42 measuring the electric current i(t) flowing through the circuit 40 serves to determine the local conductivity of the surface 31. Information is thus read in non-destructive manner.
It is important to emphasize that the variation in conductivity induced by applying the potential difference Ve(t) is localized to a region having lateral dimensions of the order of 10 nm or less, located immediately under the tip 10, thus making it possible to achieve an information storage density greater than 1 Tbit/in2. This storage density may be further increased by having recourse to non-binary coding. The variation in the conductivity of the substrate 30 is approximately proportional to the applied voltage Ve(t). If the generator 31 is adapted to apply a potential difference Ve(t) between the substrate and the tip that is selected from amongst N discrete values (not taking into consideration the low value Vr used for reading), then coding is performed on the basis of an alphabet of N symbols. For example, if N is equal to four, then an individual information storage zone can be used to record 2 bits, thereby doubling storage density compared with binary coding.
a, 3b, and 3c show the way in which the
The storage density could also be increased by making use of the conductivity σ0 as a symbol representative of information instead of serving solely to separate recording zones. Under such circumstances, the alphabet would have five symbols instead of four.
To read the information recorded in this way, it suffices to scan the surface 31 again by means of the tip 10, with the generator 41 then maintaining a constant potential difference, e.g. equal to Vr=0.3 V: the magnitude of the electric current i(t) flowing in the circuit 40, as measured by the ammeter 42, then provides the conductivity of the surface 31 and thus enables the recorded binary sequence to be reproduced.
The invention is described with reference to one particular embodiment, however numerous variants can be envisaged.
For example, the information written on the surface 31 may be read by means of a second tip that is distinct from the tip 10 used for writing. Any method of measuring the conductivity of a surface may be used for reading the information.
It is also possible to use a plurality of tips operating in parallel, both for writing and for reading information, in application of the “microtip matrix” principle shown in
Furthermore, the scanning of the surface 31 by the read/write tip(s) may be performed by moving the tip while keeping the substrate stationary, as in the above example, or conversely by moving both the tip and the substrate.
Number | Date | Country | Kind |
---|---|---|---|
07 03093 | Apr 2007 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/FR2008/000595 | 4/25/2008 | WO | 00 | 1/26/2010 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2008/145864 | 12/4/2008 | WO | A |
Number | Name | Date | Kind |
---|---|---|---|
5132934 | Quate et al. | Jul 1992 | A |
5202788 | Weppner | Apr 1993 | A |
6487106 | Kozicki | Nov 2002 | B1 |
6519221 | Manalis et al. | Feb 2003 | B1 |
20030173612 | Krieger et al. | Sep 2003 | A1 |
20040057372 | Schut et al. | Mar 2004 | A1 |
20050156271 | Lam et al. | Jul 2005 | A1 |
20050276143 | Busta | Dec 2005 | A1 |
20060153049 | Park et al. | Jul 2006 | A1 |
20060291364 | Kozicki | Dec 2006 | A1 |
20080199719 | Lee et al. | Aug 2008 | A1 |
Entry |
---|
International Search Report from International Application No. PCT/FR2008/000595, filed Apr. 25, 2008. |
Number | Date | Country | |
---|---|---|---|
20100195475 A1 | Aug 2010 | US |